This invention relates to displacement transducers, and more particularly to a method for correcting spin frequency noise errors associated with rotary transducers.
Often, it is desired to locate or measure rotary motion. Various rotary displacement transducers have been developed to convert rotary information into an electrical signal.
One type of rotary displacement transducer is a rotary encoder. Rotary encoders, also referred to as angular encoders, sense rotary displacement, convert rotary information to an electric signal, and provide a digital output, which is processed to determine angular position.
Rotary displacement transducers are used in a wide variety of applications, generalized by the occurrence of some form of rotary motion. A rotary displacement transducer is used to locate and track the rotary position of that motion. An example of a common use of rotary displacement transducers is in connection with systems that use a rotary actuator. A rotary displacement transducer is often mounted to a motor to determine shaft position. Such transducers can also be mounted to positioning tables, screw drives, gearheads, machining tools, and the like.
A problem encountered in the use of rotary displacement transducers is spin frequency error. Manufacturing tolerances and mechanical mounting differences can cause the signal from the transducer to contain anomalies. Various filtering methods could be used, but introduce phase delay and fall short of isolating only the error component or components.
One aspect of the invention is a method of correcting spin frequency error in the output of a rotary displacement transducer. For each revolution of the transducer, a series of output values is stored, each output value representing a time interval between pulses of the transducer. An xe2x80x9cinphase valuexe2x80x9d is calculated by multiplying each output value times a sin wave value, the sin wave having a period determined by the transducer resolution, and by calculating an average from the results of the multiplying step. A xe2x80x9cquadrature valuexe2x80x9d is calculated by multiplying each output value times a cosine wave value, the cosine wave having a period determined by the transducer resolution, and by calculating an average from the results of the multiplying step. The inphase and quadrature values are used to calculate a magnitude and a phase value. The magnitude and phase values are then used to calculate a series of correction values, the correction values corresponding to points on a sin wave having a period determined by the transducer resolution and having a magnitude and phase equal to the magnitude and phase values. Each correction value is subtracted from a corresponding transducer output value, thereby providing corrected transducer output values. The process is repeated for each revolution of the transducer.
An advantage of the invention is that the error correction is automatic and continuous. It removes once-per-revolution error, and any or all periodic noise components, such as a twice-per-revolution noise component, electronically and in pseudo real time. The correction process does not compromise the integrity of the measurement data provided by the transducer. In other words, there is no phase delay or magnitude coloring associated with the measurement data provided by the transducer; only the desired frequency component or frequency components of the output signal are removed and thereby affected.